Tuning the Kondo effect in Yb (Fe1−x Cox)2 Zn20

We study the evolution of the Kondo effect in heavy fermion compounds, Yb(Fe1−xCox)2Zn20 (0≤x≤1), by means of temperature-dependent electric resistivity and specific heat. The ground state of YbFe2Zn20 can be well described by a Kondo model with degeneracy N = 8 and a TK∼30 K. The ground state of YbCo2Zn20 is close to a Kondo state with degeneracy N = 2 and a much lower TK∼ 2 K, even though the total crystalline electric field (CEF) splittings are similar for YbFe2Zn20 and YbCo2Zn20. Upon Co substitution, the coherence temperature of YbFe2Zn20 is suppressed, accompanied by an emerging Schottky-like feature in specific heat associated with the thermal depopulation of CEF levels upon cooling. For 0.4≲x≲ 0.9, the ground state remains roughly the same, which can be qualitatively understood by Kondo effect in the presence of CEF splitting. There is no clear indication of Kondo coherence in resistivity data down to 500 mK within this substitution range. The coherence reappears at around x≳ 0.9 and the coherence temperature increases with higher Co concentration levels